I have this philodendron imperial green that I received about a year ago. The 2 darker green leaves have been present since I got it. In that time the plant has grown 3 new leaves and each time the same thing happens. The leaf, that’s now sitting on the right is the latest example. A new leaf forms and opens, it looks great and I get excited. Then a few weeks later it just falls off the plant, unexpectedly. Any ideas on why this happens? It gets bright, indirect sun all day and the “old” leaves plus rest of the plant seem perfectly happy.